The present invention relates to a plunger type master cylinder used in a vehicle brake system, and more specifically a simple and reliable master cylinder which can smoothly draw hydraulic fluid from a reservoir during automatic brake control and which restricts backflow of hydraulic fluid from pressure chambers into the reservoir while the brakes are being manually operated.
Many of today's vehicle hydraulic brake systems are equipped with an automatic brake function such as a traction control (TRC) function or an electronic stability control (ESC) function.
In some of such brake systems, during automatic braking, hydraulic fluid is supplied from the reservoir to required units through a pressure chamber in the master cylinder. Thus in such brake systems, it is required that hydraulic fluid be smoothly drawn into the pressure chamber of the master cylinder. Also, when the brake pedal is depressed, it is necessary to restrict backflow of hydraulic fluid from the pressure chamber into the reservoir to prevent the brake pedal from being pushed in without encountering resistance.
JP patent publication 2000-142365A (paragraph [0039] and FIG. 21) discloses an arrangement that satisfies both of these requirements. This arrangement includes a throttle valve member comprising a throttle valve portion including a restricted passage and provided on the inner periphery of a piston, and a support portion fitted in the piston to support the throttle valve portion. When hydraulic fluid flows from the pressure chamber toward the reservoir, the throttle valve portion abuts a valve seat on the inner surface of the piston, thus closing the passage between the throttle valve portion and the valve seat, so that hydraulic oil flows only through the restricted passage into the reservoir.
When hydraulic fluid flows from the reservoir toward the pressure chamber, the throttle valve portion deflects and separates from the valve seat, thus opening the passage between the throttle valve portion and the valve seat. Thus, hydraulic fluid is smoothly drawn into the pressure chamber through the passage between the throttle valve and the valve seat because this passage has a larger sectional area than the restricted passage.
However, the arrangement disclosed in this publication has a problem in that the throttle valve member is complicated in structure and thus low in productivity. In order to vent air when filling the brake system with hydraulic oil, hydraulic air is fed under pressure, or with the interior of the circuit evacuated beforehand, hydraulic fluid is dawn into the circuit. When air is vented in either of the above manners, the throttle valve portion may deflect significantly into the piston and interfere with a return spring for the piston which is mounted inwardly of the throttle valve member, thus causing deformation (plastic deformation) of the throttle valve portion. This impairs various functions inherent to the throttle valve portion.
If such throttle valve portions are provided in a tandem master cylinder, which includes two pistons mounted in the cylinder and two pressure chambers defined in the cylinder, when automatic braking begins, hydraulic fluid is rapidly returned into the pressure chambers. In this state, excessive backward movement of the primary piston (rearward piston) is prevented because it is in abutment with an output rod of a booster. But the secondary piston (front piston) may excessively move backward and come out of the cups. If this happens, when the secondary piston returns to its inoperative position thereafter, the secondary piston may damage the cups or the secondary piston may be inserted into the cups in an unnatural way and skew.
An object of the present invention is to provide a simple and reliable plunger type master cylinder which can smoothly draw hydraulic fluid from a reservoir during automatic brake control and which restricts backflow of hydraulic fluid from pressure chambers into the reservoir while the brakes are being manually operated.